When deploying wireless communications systems such as, for example, cellular systems, in indoor environments in general, traditional kinds of antennas can be less suitable to use. In such environments, use is sometimes instead made of so called “leaky cables”, also sometimes referred to as leaky feeders or radiating cables.
A leaky cable is a cable which is capable of conducting electromagnetic radio frequency energy, and which has been provided with apertures in order to make the cable radiate, i.e. to allow some of the energy to “leak” from the cable, thus enabling the cable act as an antenna. Such an antenna, i.e. a leaky cable, will due to reciprocity be able to act equally well as a receiving as a transmitting antenna. Due to its nature of a cable, a “leaky cable antenna” will, as compared to a traditional antenna, act more like a line source than a point source, thus making it easier to obtain coverage in tunnels, along railways or where a high degree of “shadowing” occurs when using a point source antenna. An example of the latter is an indoor scenario, e.g. an office landscape.
In recent years demands for high user bitrates and capacity have increased dramatically due to the growth of mobile broadband usage. In order to achieve higher user bitrates and spectrum efficiency multiple antenna techniques like Multiple Input Multiple Output (MIMO) are employed in wireless communications systems.
In deployments where multiple leaky feeders are used it is a great benefit, regarding installation, to bundle them. However, the individual characteristics of the cables may differ substantially regarding directivity. If more than two cables are bundled there might also be significant radiation efficiency differences due to mutual coupling. Azimuth antenna patterns for two cables which are bundled and extended along an axis perpendicular to the figure are shown in FIG. 1. As can be seen in the figure, a problem is that the antenna patterns only partly cover the same angular interval. A consequence is power imbalance for the different antenna branches of the leaky cables which is particularly prominent in line-of-sight conditions. The power imbalance is a problem in e.g. MIMO multi stream transmissions causing reduced capacity.